The present invention relates to stomach-action molluscicides, stomach poisons or edible baits containing them and their use in killing, controlling and/or inactivating molluses, in particular, slugs and snails.
Slugs and snails are major pests of agriculture in many parts of the world. Their biology tends to favour activity in moist conditions such as habitats which are continually wet and temperate regions, especially during rainy summers and autumns. As a consequence, their potential for damage is considerable.
The ecologies of different types of molluscs, which can be either terrestrial or aquatic, are very different and they usually require different types of treatment. The snail species Theba pisana, Cemuella virgata, Helix aspersa and Achatina spp and the slug species, Arion hortensis, Milax budapestensis, Deroceras reticulatum and Limax maximus are of particular interest as targets. The common garden snail, Helix aspersa, and the grey field slug, Deroceras reticulatum, are common garden pests throughout temperate Australia. These pests have established themselves in many parts of the world, adapting to a wide range of climatic conditions. They rarely increase in numbers above 20 per square meter but cause damage by feeding, with minor damage due to the mucus on which they move. Helix aspersa is, in general, a nocturnal feeder and in the daytime remains hidden on the underside of leaves, under rocks or in cracks in the soil. It flourishes in moist conditions. On the other hand, there are a group of snails which have been introduced into Australia in the twentieth century. The areas in which these are pests (often over 200 per square meter) are still expanding. These are the white Italian snail, Theba pisana and the vineyard or Mediterranean snail, Cernuella virgata, which can survive long hot summer temperatures by aestivating on weeds and fence posts, retreating into their shells and secreting a hard mucous film to reduce moisture loss and rest. These snails are of some concern to Australian farmers because they also aestivate on the heads of cereal stalks in November and December and during harvest, they clog up the machinery and contaminate the grain, making it either unacceptable or forcing it to be downgraded. There are very significant variations of the pest numbers and in a bad year it is uneconomic to harvest substantial areas of crops. In cold climates, Theba pisana hibernates in winter. The slug, Deroceras reticulatum, is found throughout temperate areas of the world and it is the major slug variety found in both Australia and the United Kingdom.
Significant crop damage by molluscs also occurs in northern Europe, the Middle East, North and Central America, South East Asia, Japan and New Zealand. In many cases, the rise to pest status of the slug or snail in question is a consequence of changexe2x80x94either in distribution (as in the case of accidental or deliberate introductions) or in agricultural practice, where new crops or systems of cultivation may enable populations to rise to pest levels. For example, approximately two-thirds of the molluscicides in the United Kingdom are used on winter wheat and winter barley. After harvesting, there is a significant amount of stubble left behind. It is present agricultural practice to drill seeds of the next crop directly into the soil, without removing the stubble of the previous crop by, for example, burning. Slugs, which have buried themselves in the soil, move along into these drill holes and eat the inside out of the new seed, thereby potentially destroying the whole planting. Slugs are therefore a major agricultural pest.
Devising methods of controlling these pests presents a formidable task. Control methods involve cultivation practices, chemical and biological methods. Cultivation procedures that remove or make the habitat of the mollusc less attractive, are usually less expensive. Biological control by introduction of natural predators is a preferred method because, in principle, the predator could be snail specific and not harm native snails or non-target organisms. However, very extensive testing is required and, once predators have been introduced, it is very difficult to reverse the process and to remove them. Chemical methods (molluscicides) involve the use of a contact or stomach poison, an irritant or a feeding depressant.
The environment which the mollusc inhabits is generally treated with the molluscicide which is then ingested by the mollusc. Since most snails and slugs thrive in moist conditions, any effective molluscicide should be effective under these conditions. This feature of appropriate water resistance has major implications in broad-acre agriculture, where one treatment is preferred rather than multiple applications throughout the crop season. In this case, it is desirable to have a balance between water resistance and efficacy to prevent the pellets functioning as poisons after the crop has been harvested and livestock has moved into the area to feed. In addition, in areas of very high moisture content there should be effective water-proofing to ensure the poison is maintained in an ingestable form for a sufficient time to permit adequate exposure to the molluscs. Since moisture is essential for slug and snail activity, damage is likely to be more severe on heavy soils due to their greater moisture retention. However, damage is not restricted to heavy soils. Slug and snail activity is encouraged by high levels of organic matter which often provides a moist environment. Green manure crops and old crop residues used in the compost heap often allow populations to build up quickly. Dense leafy plants, such as brassica and curcubitis, provide a moist humid canopy under which snails and slugs thrive. Temperature also affects the level of slug and snail activity. Indeed, this activity peaks around 15-20xc2x0 C. and decreases markedly below 5xc2x0 C. and above 30xc2x0 C. Furthermore, low temperatures significantly delay the hatching of slug eggs. Most slug and snail species are nocturnal feeders. Hence, watering of gardens in the evening often provides an environment which encourages increased feeding activity.
Molluscicides for use against slugs and snails can be divided into three groups. These are contact-action molluscicides, such as aluminium and copper sulfate crystals, which are applied to the area inhabited by the snail or slug and are taken up passively when the snail or slug moves in this area; irritant powder molluscicides, such as silica grains, which act by being taken up in the snail""s or slug""s locomotion mucus; and stomach-action molluscicides such as metaldehyde and methiocarb pellets, which are ingested by the mollusc.
Contact-action molluscicides are generally applied in the form of sprays and dusts to crops and the mollusc receives a fatal dose of toxin by moving over the crop. Molluscs present problems of delivery of the toxin because their relatively large size means that a large dose of toxin is necessary. They are also relatively inmobile and may remain concealed in comparative safety for long periods. These problems are further complicated by the layer of mucus which invests molluscs. Irritant materials stimulate mucous production and can be sloughed off and left behind in a discarded mucous coat. As the mucus is largely composed of water, the water-solubility of candidate contact poisons is therefore a prerequisite if they are to be able to penetrate the mucous barrier. However, hydrophilic properties in a toxin also increase the rate at which it is diluted by rain and leached into the soil.
Delivery of effective amounts of bait is also a problem. A sufficient amount of poison must be ingested to ensure a lethal dose. In general, most toxic compounds are also repellent and the interaction of toxicity with repellency prevents the ingestion of sufficient poison to kill the mollusc. There are three major effects of molluscs ingesting poison baits. Firstly, there is a possible repellency away from the crop by the bait. Secondly, ingestion of the bait may cause reduced feeding and thirdly, the poison may kill the snail or slug involved.
Until the mid 1960""s, the most effective molluscicide was metaldehyde which is a tetramer of acetaldehyde. In Europe, it was known only as a solid fuel, until its molluscicidal properties were discovered accidentally in France by farmers who found dead and dying snails on and around metaldehyde tablets discarded after use in camping stoves. Metaldehyde is toxic at high concentrations and an irritant at lower concentrations, causing mucous secretion and eventual desiccation. A disadvantage is its dependence on high temperature and low humidity for its maximum effect and there is a high recovery rate amongst molluscs which are able to reverse the water deficit caused by the excess mucous secretion that metaldehyde stimulates. Under optimal conditions, slugs immobilised and desiccated by metaldehyde will not survive if trapped in the open and exposed to sunlight. Unfortunately, it is under damp conditions and at lower temperatures when metaldehyde is least effective that terrestrial slugs and snails are most active and yet, at higher temperatures, snails are aestivating and not feeding. There is only a very limited period of time during which snails are feeding and the temperature is high enough for metaldehyde to be effective.
In the mid 1960""s, it was found that carbamate compounds such as methyl carbamate were as toxic to molluscs as metaldehyde. Carbamate compounds cause inhibition of cholinesterases which are the enzymes involved in synaptic nervous transmission in a wide range of animals and their mode of action on insect pests has been extensively studied, particularly in connection with the development of resistance. The methyl carbamate most widely used as a molluscicide is methiocarb (3,5-dimethyl-1,4-methylthiophenyl-N-methylcarbamate). The effectiveness of methiocarb is compromised less by low temperatures and high humidity than metaldehyde which is a major advantage, since pest damage often occurs in conditions where metaldehyde is least well suited. However, methiocarb (an active insecticide and acaricide) is more toxic to non-target organisms such as beneficial insects and earthworms than metaldehyde. Although farmers presently tend to use methiocarb, they would prefer not to because of these highly poisonous characteristics and the fact that sheep often graze in areas that require treatment for snails and slugs. For example, in South Australia there are flood-irrigated pastures for sheep and recently, a high incidence of the conical variety of snail, Cochlicella barbara, has been detected. Therefore, any effective molluscicide used under these conditions would have to be effectively water-proofed in addition to not being toxic to the sheep. Methiocarb is effective on Theba pisana but in view of its insecticidal activity and toxicity to earthworms, its use for this snail variety also has severe drawbacks.
There is considerable evidence to indicate that metal salts used as contact poisons are toxic to molluscs (Glen, D. M. and Orsman, I. A., in xe2x80x9cComparison of molluscicides based on metaldehyde, methiocarb and aluminium sulphate,xe2x80x9d Crop Protection, (1986), 5, 371-375.) In particular, iron and aluminium salts have been investigated in this regard in some detail in the United Kingdom (Henderson et al, xe2x80x9cAluminium(III) and Iron(III) complexes exhibiting molluscicidal activity,xe2x80x9d Australian Patent AU-B-22526/88). These workers concluded that the effectiveness of the molluscicide was dependent on a number of variables but the chelating of the trivalent iron gave very significantly better results than the unchelated salts. In addition, these workers found that the inclusion of the poison in a bait, as a pellet, gave significantly better results than the direct application of molluscicide to the soil or application of the bait as a powder to the soil. Details of the bait formulation were given without discussion of differences that might be expected from other formulations. Such differences are most probably significant in determining the amount of chelate required for effective control. In field conditions, the efficacy and activity of many metal salts is greatly attenuated by both dilution and the metal ions becoming chemically bound in the soil and being unavailable for toxic action. Proposed contact-action metal poisons such as aluminium tris(acetylacetonate) (xe2x80x9cAl(acac)xe2x80x9d)3 are expensive to manufacture and are therefore not economically feasible for use in the home garden or for horticulture or broad-acre application. Various metal salts are marketed as contact molluscicides and are indeed toxic, but it is debatable whether they are effective under field conditions. As contact-action poisons, they are insufficiently persistent and too repellent to be used in baits. For these reasons, molluscicides used against terrestrial (as opposed to aquatic) targets are usually delivered in the form of stomach-action poisons in baits.
One of the other major problems with stomach-action poisons in that they are often consumed by non-target organisms such as domestic animals, birds and children. In normal agricultural and veterinary applications, the preparations are usually very dilute when applied. However, when baits are used this is not the case and there is always a possibility that the bait will be consumed by a non-target organism Accidental poisoning of non-target organisms is particularly common in the case of snail and slug bait pellets. It is hard to arrive at a reliable figure for poisoning of dogs, cats and native animals, but in Australia about 10,000 poisonings per annum with perhaps as high as 40-50% being fatal is probably a reasonable estimate. A requirement accordingly exists for molluscicides which are effective against snails and slugs, but which substantially minimize the health and environmental risks and cost limitations of the molluscicides currently available on the market.
There are a number of published efficacy trials which indicate that Ferric sodium EDTA (Iron(III) EDTA or ferric EDTA) salt is an effective contact-action molluscicide. Research has been conducted on a number of iron and aluminium compounds as contact poisons against the slug Deroceras reticulatum (Henderson, I. F. and Martin, A. P., in xe2x80x9cControl of slugs with contact-action molluscicides,xe2x80x9d An. Appl. Biol., (1990), 116, 273-278). These workers reported two types of experiments, one in which the slugs were confined to a treated glass surface and one using wet soil in a laboratory test. Unchelated salts were effective poisons when applied to a glass surface, but were rapidly deactivated when applied on wet soil. Chelation of both metals with organic ligands retarded the rate of attenuation on wet soil. These workers also reported a field trial in which chelated iron in a broadcast application applied at 40 kg active ingredient per hectare, or in a bait formulation applied at 1.32 kg/ha of active ingredient was effective against Deroceras reticulatum and Arion spp. They concluded that xe2x80x9con the available evidence, the bait formulation was apparently more efficient, an application rate of 1.32 kg active ingredient leaving 586 slugs dead on the surface within three days while with the broadcast formulation applied at 40 kg active ingredient per hectare, only 204 were recorded dead on the surface in the same period.xe2x80x9d Iron(III) 2,4-pentanedionate appears to be more toxic than Iron(III) EDTA and although it is difficult to quantify the difference, it appears that on wet soil after 10 days, the 2,4-pentanedione is about twice to three times as toxic. Details of the bait formulation were not given but these are most probably significant in determining the amount of chelate required for effective control.
The inclusion of metal chelates as the active ingredient in stomach-action poisons in accordance with the invention, however, offers considerable advantages over the presently used stomach-action molluscicides, metaldehyde and methiocarb. The present invention concerns the inclusion of a complexone as the chelating ligand to function as the active ingredient in stomach-action poisons. Selected complexones are considerably less toxic to mammals than methiocarb or metaldehyde. Indeed, they are used in medical applications to relieve anaemia. Such complexones are often used in trace-element mixes in situations where a plant is suffering from an iron deficiency. The effectiveness of such complexones is not very temperature- or humidity-dependent, being comparable with methiocarb in this respect. They are neither insecticides nor acaracides and snail and slug pellets based on such compounds will not kill earthworms or the (mainly beneficial) carabid beetles. The term xe2x80x9cmetal complexonexe2x80x9d is used herein in its broadest sense and refers to a chelate of a metal with at least one ligand of the complexone type.
The term xe2x80x9ccomplexonexe2x80x9d as used herein refers to an organic ligand containing at least one iminodiacetic group xe2x80x94N(CH2CO2H)2 or two aminoacetic groups xe2x80x94NHCH2CO2H which form stable complexes with most cations. Suitable complexones include those disclosed in Wilkinson, G., xe2x80x9cComprehensive Coordination Chemistryxe2x80x9d, Volume 2, Chapter 20.3, pp 777-792 which is incorporated herein by reference.
Preferably, the complexone is a derivative of ethylenediaminetetraacetic acid (hereinafter referred to as xe2x80x9cEDTAxe2x80x9d) as shown in formula (I) below: 
wherein n is an integer, preferably from 1 to 6.
Other examples of suitable complexones include those having more than four acetic acid residues as shown in formula (II) below: 
wherein n is an integer, preferably from 1 to 3 or as shown in formula (III) below: 
wherein n and m are integers, preferably from 1 to 4.
While the major complexone utilized in the present invention is EDTA, other complexones, in particular, those having substituents such as hydroxy groups which coordinate to the metal ion more strongly, such as EDDHA, or those which display increased stability due to the presence of an additional coordinating group, such as DPTA, have also been utilized in the trials. Other chelates investigated include ferric sodium ethylenediaminebis[(2-hydroxyphenyl)acetic acid] (xe2x80x9cFeEDDTAxe2x80x9d) and ferric sodium diethylenetriaminepentaacetic acid.
According to one aspect of the present invention, there is provided a stomach-action molluscicide which includes a metal complexone as an active ingredient.
Preferably, the metal complexone includes hydroxy- and non-hydroxy-metal complexones. Most preferably, the active ingredient is an hydroxy-metal complexone.
Typically, the stomach-action molluscicide has a pH above 7. Preferably, the pH is between about 7 and 10. Most preferably, the pH of the molluscicide is about 8.
Typically, the metal of the metal complexone is selected from the group of Group 2 metals, transition metals or Group 13 metals. Preferably, the metal is selected from the group of magnesium, aluminium, manganese, iron, copper or zinc.
Preferred metal complexones include iron(II) and iron(III), copper and zinc EDTA. Iron EDTA and copper EDTA are preferred with iron EDTA being the most preferred. Iron EDTA is also not harmful to the environment as it is widely used as a source of iron for plants and to a limited extent animals in horticulture and agriculture.
Typically, the complexone comprises at least one iminodiacetic group or two aminoacetic groups, the complexone forming a stable complex with the metal. Preferably, the complexone has at least four acetic acid groups. More preferably, the complexone is ethylenediaminetetraacetic acid (EDTA). Most preferably, the active ingredient in the molluscicide is the hydroxy-metal complexone, [Fe(OH)EDTA]Ca which can dimerise to give [EDTAFexe2x80x94Oxe2x80x94FeEDTA].2Ca. Anions [Fe(OH)EDTA]2xe2x88x92 and [EDTAFexe2x80x94Oxe2x80x94FeEDTA]4xe2x88x92 are important species and it appears that the inclusion of Ca2xe2x88x92 is advantageous as it eventually replaces the chelated iron.
In a preferred form of the present invention, the molluscicide is advantageously presented in the form of a stomach poison together with a carrier. The carrier usually includes a mollusc food such as a cereal, for example, wheat flour, bran, arrowroot or rice flour, carrot; beer; rice hulls; comminuted cuttle fish; starch or gelatin so that the mollusc is attracted to the edible bait. Non-nutrient carriers of interest include non-nutrient polymeric materials, pumice, carbon and materials useful as carriers for insecticides. The poison or bait may also contain other additives known in the art such as mollusc phagostimulants for example sucrose or molasses; lubricants such as calcium or magnesium stearate, talc or silica; binders which are suitably waterproof, such as paraffin wax, white oil or casein; and flavouring agents such as BITREX(copyright) which imparts a bitter taste and renders the poison or bait less attractive to non-target organisms. In order to inhibit deterioration of the poison or bait, preservatives such as sodium benzoate, vitamin E, alpha-tocopherol, ascorbic acid, methyl paraben, propyl paraben or sodium bisulphite may also be included. Preferably further, the waterproofing agent comprises a fatty acid alcohol in an amount about between 1% to 5% by weight of the total composition of the poison. More preferably, the fatty acid alcohol is selected from the group of C16-C18 fatty acid alcohols. Most preferably, the C16-C18 fatty acid alcohols comprises about 5% by weight of the total composition of the poison and the C16-C18 fatty acid alcohol is HYDRENOL MY manufactured by Henkel Corporation. The waterproofing of the bait is also believed to be improved if the density of the bait is increased since the less porous the composition, the more effectively waterproofed it becomes.
To improve the density of the acual mixture before pelletising to reduce the airborne content and thus wastage of the mixture, a filler is added to the carrier. Preferably, the filler is either CaCO3 or K2CO3. Typically, the poison or bait contains about above 1% and not exceeding 5% of a metal carbonate as a filler. When the metal carbonate is CaCO3, the preferred concentration is about 2-3% by weight. When the metal carbonate is K2CO3, the preferred concentration is about 4-5% by weight. A combination of CaCO3 and K2CO3 may also be used.
Serendipitously, such a metal carbonate additionally serves to adjust the pH of the poison or bait and it was found that the efficacy increased with an increase in pH. It was found through trials carried out using various amounts of CaCO3 and K2CO3 combined, that a balance needs to be struck between the pH and the attractiveness of the bait to the molluscs. If the bait is too acidic, it has been found that the efficacy is reduced. Conversely, if the bait is too alkaline this also deters feeding. Typically, the poison or bait has a pH about above 7 and not exceeding 10. Preferably, the pH of the poison or bait is about 8. Preferably, the agent used to adjust the pH is K2CO3 together with CaCO3. A stomach poison having a neutral or alkaline pH has proved to be more efficacious than one having an acidic pH. The K2CO3, together with the CaCO3, used as a filler and which adjusts the pH to about above 8, aids in the formation of the active ingredient, [Fe(OH)EDTA]Ca. The person skilled in the art will appreciate that the behaviour of ferric EDTA in solution and at equilibrium is strongly determined by its speciation. At a pH of between 7 and 10, the species present in the majority is [Fe(OH)EDTA]2xe2x88x92 with [Fe(III)EDTA] being present in the minority. According to F. G. Kari et al, Environ. Sci. Technol., (1995), 29, 1008, at a pH of about 8 to 8.5, there is virtually no [Fe(III) EDTA] species present at all.
Preferably, the active ingredient comprises at least 6% by weight of the total composition of the molluscicide. More preferably, the active ingredient comprises about 6% to about 12% by weight of the total composition of the molluscicide when the active ingredient is [Fe(OH)EDTA]Ca or its dimer[EDTAFexe2x80x94Oxe2x80x94FeEDTA].2Ca. Most preferably, [Fe(OH)EDTA]Ca comprises about 9% by weight of the total composition.
According to yet another aspect of the invention, the active ingredient comprises a metal complexone in combination with at least one other molluscicide. Typically, the other molluscicide is selected from metaldehyde or methiocarb, wherein the other molluscicide is in a synergistic relationship with the metal complexone.
The molluscicide is advantageously presented in a solid form such as tablets, powders, granules or pellets. Those skilled in the art will appreciate that it is preferable to prepare the products the subject of the invention in a form that is easy for consumers to use. Pellets, for example, can be easily scattered from a box across the area to be protected. Preferably, the molluscicide is in the form of a pellet. More preferably, the pellet is between 2.5 and 4 mm long. Most preferably, the pellet is 3 mm long.
According to another aspect of the invention, the method of preparation of the stomach-action molluscicide in pellet form includes the steps of:
(i) blending the molluscicide and elements of the carrier together to form a blended composition;
(ii) heating the blended composition for about 1 to 5 minutes in the presence of steam at an ambient temperature of between about 80 and 100xc2x0 C.;
(iii) maintaining the bended composition at the ambient temperature for between 10 and 30 seconds; and
(iv) forming the blended composition into one or more pellets.
Preferably, step (ii) is carried out at about 90xc2x0 C. for about 2 minutes, whereafter step (iii) is carried out for about 15 seconds. Preferably, the blended composition is formed into pellets by extrusion.
The term xe2x80x9cstomach-action molluscicidexe2x80x9d is used herein in its broadest sense and includes a molluscicide which is capable of being ingested into the stomach of the mollusc in an effective amount so as to kill and/or inactivate the mollusc.